Not applicable.
Not applicable.
This invention relates to the field of paper production, more specifically in the treatment of spent digestion liquor resulting from the production of paper pulp.
Paper manufacture requires a source of cellulose fibers. Common raw materials as cellulose fiber sources are hardwoods and softwoods as well as those of annual vegetable origin, such as wheat and rice straw, bagasse (sugar cane stalks after processing), hemp, and jute. Rag materials, as well as recycled fibers, can also be used. However, wood has been, and continues to be, a primary source of cellulose fibers for paper production.
Before use, the wood or other cellulose containing raw material must be processed to release the cellulose fibers. This operation is referred to as xe2x80x9cpulpingxe2x80x9d. At present, commercial pulping operations are of three principal types: mechanical, full chemical, and semichemical. The processes with which the invention is concerned are full chemical and semichemical pulping based on alkali, typically soda, breakdown of the fiber source. Another method of pulping, known as sulfite pulping, involves chemical transformation of the lignin component of the fiber source resulting in chemically modified lignin to be solubilized in water. Sulfite pulping is generally excluded from the inventive process to be described herein.
Full chemical and semichemical pulping employ chemical reagents to effect separation of the cellulosic fibers from other components. Wood chips or other raw materials are cooked with suitable chemicals in aqueous solution, usually at elevated temperatures and pressures. The object is to dissolve the organic binders holding the cellulosic fibers, termed xe2x80x9cligninsxe2x80x9d, which comprise up to 26% of raw wood, for example, along with other types of organic molecules, such as saccharide molecules, and other extraneous compounds, leaving the cellulose fibers intact. Although there is generally some resulting cellulose degradation, the objective can be realized to a commercially satisfactory degree through the use of a variety of chemical reagents. Pulp yields from wood using such processes are usually about 50% of the wood weight.
Lignins have been studied extensively and are believed to consist of the noncarbohydrate portion of the cell walls of plant materials. Originally, the lignin content of plant materials was defined as the residue after hydrolysis with strong acid following removal of waxes, tannins, and other extractives, including resins and tall oils. Lignins are amorphous, have high molecular weight, and are predominantly aromatic in structure. In general, the monomeric units comprising lignins can be referred to as p-hydrocycinnamyl alcohols. More specifically, according to The Merck Index, lignins comprise coniferyl, p-coumouryl, and sinapyl alcohols. Their precise composition vary with several factors including the method of isolation, and the species, age, and growing conditions, of the plant. Lignins are more or less completely removed from fiber by the digestion process of chemical pulping and enter the water phase of digestion liquor in oxidized colloidal form.
Digestion liquors obtained from alkaline pulping usually contain not only all the lignins in the source material, but substantial amounts of cellulose or carbohydrate monomers, and other carbohydrates. When using annual plant materials as a cellulose source, such as rice and wheat straw, a significant percent by weight of the material is silica. Such used or spent digestion liquors, normally waste, pose problems that are unique in alkaline pulping operations. Waste liquor streams from other operations during the paper making process pose different problems, such as removal of the resins and tall oils found in gymnosperm trees.
The lignin solids precipitate when the spent alkaline digestion liquid is acidified. This process poses a particular problem, because the lignin solids are polymerized by acidification to produce an amorphous gum.
Soda and sulfate pulping are both known in the art as being alkaline pulping processes. The soda process employs caustic soda (sodium hydroxide), whereas the sulfate process employs sodium sulfide in addition to caustic soda. The sodium sulfide used in the sulfate process results in a stronger cooking liquor and accounts for stronger pulp and faster cooking in the sulfate process as compared with soda pulping. The term xe2x80x9ckraft pulpingxe2x80x9d is an alternative to the term sulfate pulping. For the purposes of this invention, there is no practical difference between the lignin-laden, spent liquors that result from either the sulfate or the soda process or from semichemical or other pulping processes which make use of alkaline agents in conjunction with mechanical means to make pulp, except there is often a relatively great amount of silica in soda process digestion liquor as compared with sulfate (kraft) process digestion liquor. The common link between pulp-making processes with which the invention is concerned is that spent digestion liquors employing alkali, whether buffered or not, become laden with solids and with organic matter, usually referenced to as total organic carbon [TOC], primarily lignins, and that both the inorganic and organic constituents must be recovered or otherwise processed to accommodate environmental concerns as well as to recycle inorganic digestive chemicals. All cooking or pulping reagents employing alkali chemicals, especially caustic soda and sodium sulfide, are expensive. Moreover, the inorganic waste materials are usually too toxic to release the spent liquor to the environment.
The sulfate, (e.g. kraft process) and the semikraft process are generally used when wood is the raw material. The active pulping ingredients, sodium hydroxide and sodium sulfide, make up a strongly alkaline solution. Standard in the kraft pulping process is the provision of a liquor-recovery cycle in which the organic constituents in the spent digestion liquor (primarily residual lignins and carbohydrates) are burned for steam generation and for recovery of the inorganic, alkaline, pulping chemicals in molten form, they being then solubilized by the addition of water to form so-called xe2x80x9cgreenxe2x80x9d liquor, which is further processed for reuse.
The traditional waste digestion liquor recovery cycle applied most frequently to kraft or semikraft process digestion liquors comprises the step of evaporating digestion waste liquor, the so-called xe2x80x9cblack liquorxe2x80x9d, to a high concentration, to so-called xe2x80x9cconcentrated black liquorxe2x80x9d or xe2x80x9cblack kraft liquorxe2x80x9d, which is usually (up to) 70% solids, or greater, by weight. Organic sulfur compounds are found in the black liquor from the sulfate process in association with sodium sulfide (NaS). Sodium carbonate (Na2CO2), sodium sulfate (Na2SO4), and silica (SiO2) are also present. Total solids are usually about 15 percent by weight in black liquor after separation from fiber pulp following digestion.
The term xe2x80x9cblack liquorxe2x80x9d is often also applied to other lignin-laden, used or spent digestion liquors, the compositions of which vary with the reagent chemicals used, the raw material, and the particular mill concerned.
The soda process is normally applied to raw materials of annual vegetable origin, such as cereal (e.g. wheat and rice, straw). Such materials normally contain a relatively high percentage of silica, which is solubilized in the digestion liquor. This poses additional separation problems, because, as well known in the art, separating out silica by acidification of the alkaline spent digestion liquor produces a gelatinous or gummy mass that cannot be separated from the liquor in a practical manner. The elevated silica content of liquor derived by pulping such vegetable fiber sources, as much as one percent by weight, as compared to the relatively low silica content from wood fiber sources, generally precludes practical application of separation and recovery methods presently known to those skilled in the art.
In the usual kraft recovery process in which silica is a negligible factor, after the black liquor is evaporated to about 70% by weight solids, other procedures, such as vacuum flashing, may be performed to increase even more the preparation of solids for burning. The high-solids-content, kraft black liquor is fed into a reducing recovery furnace provided as part of the usual kraft pulping plant for chemical and energy recovery. The usual reducing recovery furnace requires a large capital investment, and its capacity frequently limits production from a typical kraft pulping plant.
Some alternatives to furnace recovery have been disclosed. U.S. Pat. No. 3,546,200 to David M. Whalen et al disclose a simple method for precipitating lignin from kraft black liquor. In the disclosed method, kraft black liquor is added slowly and with stirring to a mixture of an organic liquid, such as chloroform, and enough mineral acid to bring the final pH to about 3. Large amounts of organic liquid required by the disclosed process make the process impractical on a commercial scale.
U.S. Pat. No. 6,632,327 to El-Shall discloses mixing a water soluble, surface active, polymeric agent with digestion liquor, preferably with an additional surface active defoamer coagulant agent prior to or during acidification of the alkaline digestive liquor. This results in an easily separable, non-gelatinous, non-gummy, coagulated lignin as a solid, particulate faction that tends to float on a clarified liquid fraction containing recoverable salts. The solids faction and the liquid frictions are easily separated, one from the other, such as by a gravity separation step. U.S. Pat. No. 6,632,327 to El-Shall is incorporated by reference into the present application in its entirety.
Although U.S. Pat. No. 6,632,327 to El-Shall represents a major advance in treatment of spent alkaline digestion liquor from paper pulping operations, a more efficient and economical way of recovering alkaline pumping chemicals is still needed.
A method of treating spent alkaline pulping liquor (black liquor) includes the steps of providing black liquor having lignin, and acidulating a phosphate containing material, wherein phosphoric acid is generated by acid treatment of the phosphate containing material. The black liquor is then mixed with phosphoric acid from the acidulated phosphate containing material to cause separation of lignin from the black liquor, wherein clarified liquor comprising anion species including phosphate is produced.
The phosphate containing material can comprise phosphate comprising rock, wherein the acidulating step comprises contacting the phosphate comprising rock with a sulfuric acid solution to form an acidulated phosphate rock slurry. The amount of said sulfuric acid solution used in the acidulation step is preferably less than the amount necessary for complete conversion of the phosphate rock or other phosphate containing material to phosphoric acid so that there is no measurable sulfate ion concentration remaining in the clarified liquor. By acidulating with a quantity of sulfuric acid less than the stoichometric amount, it is assured that sulfate will be consumed by the excess of phosphate in the rock, and thus no measurable sodium sulfate will be present in the clarified liquor. This condition is generally preferred because recovery of sodium hydroxide from sodium sulfate is not possible using currently available cost effective methods.
In a preferred embodiment of the invention, the method includes the step of adding a water soluble, surface active, polymeric, coagulant agent to the black liquor. The polymer addition is generally most effective when applied to the black liquor prior to mixing with the acid. A surface active agent coactive with the polymeric, coagulant agent, can also be added to the black liquor prior to the mixing step. The method can include the step of separating (e.g. filtering) the acidulated phosphate containing material to form a solid-free liquid phase comprising phosphoric acid, then using the phosphoric acid produced in the mixing step.
The method preferably includes the step of causticizing the clarified liquor using a material comprising calcium oxide. The causticizing step produces calcium phosphate precipitates, such as calcium monophosphate, calcium monophosphate being an agricultural fertilizer.
The black liquor can be obtained from a kraft process pulping plant or a soda process pulping plant. Thus, the invention provides black liquor treatment using low cost acid (e.g. sulfuric acid) and low grade phosphate containing material (e.g. rock) to, achieve the equivalent pulping chemical recovery that could only be heretofore achieved using high cost phosphoric acid.